ABSTRACT
The interplay of multiple genes and environmental factors generates interindividual variation in plasma low density lipoprotein-cholesterol (LDL-C) concentrations. As a result, it has been difficult to identify individual genes that contribute to variation in plasma LDL-C levels using classical linkage analysis. We have exploited a genetic defect in the gene encoding the LDL receptor that is associated with a dramatically elevated plasma LDL-C level to unmask an allele at another locus that lowers plasma LDL-C levels. The existence of such an allele was implied by the analysis of a human pedigree with familial hypercholesterolaemia in which a third of the familial hypercholesterolaemia heterozygotes had normal levels of LDL-C. To develop an animal model of this LDL-C lowering effect and to identify genes that modify the plasma LDL-C level, we crossed LDL receptor-deficient mice with other strains of mice.